Single-Celled Life

Eukaryotes and Prokaryotes were the first life forms on earth. They represent the beginning of life, and were the dominant organisms for billions to millions of years. These such organisms still exist, albeit not as common as they used to, those are extinct. Fossils have been found from the prehistoric bacteria, mostly in the Late Archean and Early Proterozoic. They were dominant starting 3.8 billion years ago to the Tonian Period, 1 billion years ago. They went extinct (most) due to the increase in multi-cellular organism such as sponges and Dickinsonia in the Cryogenian and Ediacaran periods.